Conventionally, electrophotographic image forming apparatus (e.g., copiers and laser beam printers) are configured to rotate the photoconductor drum for evenly charging the entire surface of the photoconductor drum. An electrostatic latent image is then formed on the photoconductor drum by sweeping a laser beam over the photoconductor's surface. A toner image, an image developed by a toner deposited on the electrostatic latent image, is transferred onto a sheet and is fixed to the sheet by a fixing device.
Fixing devices used for this purpose include heating roller fixing devices. The heating roller fixing include: directly heating a target sheet by a heating roller; heating a fixing belt by a heating roller followed by heating the target sheet by the heated fixing belt; and so forth.
A heat generating lamp, a heat generating source, is incorporated inside the heating roller.
In recent years, it has been required in the art for fixing devices to be capable of accommodating multiple sheet sizes while consuming as little electric power as possible and of obtaining high image quality through a uniform fixing temperature distribution.
Heretofore, configurations in which a plurality of heat generating lamps are provided in the heating roller in order to accommodate a plurality of sheet sizes are disclosed in, for example, Patent Literatures 1 and 2.
Patent Literature 1 discloses a fixing device that includes a plurality of heating lamps for different sheet sizes.
Patent Literature 2 discloses a fixing device that includes a first heat generating lamp having a heat generating region at a position corresponding to the center and its vicinity of a sheet in order to heat the central region of the sheet, and a second heat generating lamp having a heat generating region at positions corresponding to the edges and their vicinity of the sheet in order to heat the edge regions of the sheet. The fixing device turns on only the first heat generating lamp when a small-sized sheet is fed, and turns on both the first and second heat generating lamps when a large-sized sheet is fed. Consequently, the fixing device can accommodate a plurality of sheet sizes.